Magnetic bearing apparatus and rotating machine having such an apparatus

Abstract: A magnetic bearing is provided with a simulating circuit (10) for estimating a displacement of a rotor (1) on the basis of a force which is exerted on the rotor (1). An output of the simulating circuit (10) is compared with an actual displacement signal. A delay circuit (13) and a subtracter (21) are provided to subtract a run-out which was estimated one period before the rotation of the rotor from the displacement signal. A displacement signal from which the run-out component was eliminated is set to a new displacement signal. A signal which is obtained by adding an estimation run-out of one period before, namely, an output of the delay circuit to the run-out which was estimated by comparing the output of the simulating circuit (10) with the new displacement signal is newly inputted to the delay circuit (13). Due to this, the run-out component is eliminated from the displacement signal of the magnetic bearing, a load of power amplifiers (7a, 7b) for driving electromagnets (3a, 3b) is reduced, there is a surplus for a saturation which is caused in a high frequency band, the reliability of a control system is improved, and a deterioration of a positional accuracy which is caused by the run-out is prevented.

Description: The present invention relates to a magnetic bearing apparatus having a magnetic bearing for rotatably supporting a rotor by using a magnetic force. A magnetic bearing according to the invention is an active magnetic bearing for supporting a rotor in a space by an attracting force of electromagnets which face each other through the rotor. To stably support the rotor, the position of the rotor is always monitored by a displacement sensor and an electromagnetic force of an active type magnetic bearing is controlled on the basis of an output signal of the displacement sensor. A compensating circuit which is used in the control of the magnetic bearing is generally fundamentally based on a PID control. Fig. 7 shows a concept of a radial bearing as one of the magnetic bearings. Reference numeral 1 denotes a rotor; 2 a displacement sensor; 3a and 3b electromagnets; 4 a converter of the displacement sensor; 5 a compensating circuit; 6 a positive/negative switcher; 7a and 7b power amplifiers (current amplifiers); and 8 an auxiliary bearing. The control of such a magnetic bearing has been disclosed in "Dynamics of Apparatuses Using Electromagnetic Force", Corona Co., Ltd., published on 25 July, 1990, pages 159 - 176, and Hannes Bleuler, "Decentralized Control of Magnetic Rotor Bearing Systems", (thesis for a degree of a Switzerland College of an Institute of Technology), Juris Druck + Verlag Zurich, 1984. An apparatus for taking a countermeasure for an overload at the time of a high speed rotation in case of using the magnetic bearing is disclosed in JP-A-4-8911. In the above conventional magnetic bearing, in order to assure a rigidity or damping property of the bearing, it is necessary to set a gain of a compensating circuit to a large value. Such a point is particularly, remarkable in a high frequency region. In this case, noises cannot help being amplified. On the other hand, since a load of a power amplifier is an electromagnet, namely, inductance, a current control ability is limited in the high frequency region in which the impedance increases. When a current command over the ability is given to the power amplifier, since the ability of the power amplifier is saturated, phase characteristics are deteriorated and it is difficult to control the magnetic bearing as a whole system and instability is enhanced. To avoid such a situation, therefore, it is necessary to prevent that a current command in a high frequency range such as noises or the like is inputted to the power amplifier. As one of the causes of the high frequency noises, there is a run-out which is captured by a displacement sensor. As a narrow meaning, "run-out" means noises which are generated in a signal of the displacement sensor due to a distortion of the surface shape of the rotor. As a broad meaning, "run-out" means noises which also include a whirling signal which is caused in the signal of the displacement sensor due to a deforming shape of the rotor. In the specification, the former one is called a "run-out". The noises due to the run-out include the noises which are synchronized with the rotational speed and their double harmonics. As a diameter of the machine increases and the machine rotates at a high speed, such a noise problem becomes typical and its countermeasure is required. An apparatus for attracting a rotary shaft in one direction when an output of the displacement sensor is equal to or larger than a set value has been disclosed in JP-A-4-8911. In such a conventional apparatus, however, there is an inconvenience such that it is necessary to attract the rotary shaft by an auxiliary magnet and the apparatus increases in size. The object of the present invention is to provide a magnetic bearing apparatus with improved control without making a construction of the rotating system complicated and a preferable use of such a magnetic bearing apparatus. According to the invention this object is achieved by a magnetic bearing apparatus according to claim 1 and by using of such a magnetic bearing apparatus in a rotating machine according to claim 7. The improved control in the magnetic bearing apparatus according to the invention is achieved by eliminating noises which are caused by the run-out. Since the run-out component is eliminated from the displacement sensor signal of the magnetic bearing, a load of the power amplifier to drive the electromagnet is reduced, there is a surplus for the saturation which is caused in a high frequency band, and the reliability of the control system can be improved. A positional accuracy which was deteriorated due to the run-out can be improved. The simulating circuit is adjusted so as to simulate a transfer function (compliance) of the rotor at a high fidelity. The force which is exerted on the rotor by the magnetic bearing is inputted as information and an estimated displacement of the rotor is generated. On the other hand, the run-out which is caused due to a decentering of a sensor target or a working roughness is included in the output of the displacement sensor which is actually attached in the machine. However, by substracting the output of the simulating circuit, the run-out component is extracted. Preferred embodiments of the magnetic bearing apparatus according to the invention are subjects of claims 2 to 6. The estimated run-out is delayed by the delay circuit by a time of only one rotation period and is used to eliminate the run-out from the output of the displacement sensor. A mechanism in such a case will now be described in detail hereinbelow. First, the run-out is estimated at the time of the first rotation. At the time of the subsequent second rotation, the estimated run-out at the first rotation time is subtracted from the original output of the displacement sensor and the result is inputted to a compensating circuit of the magnetic bearing. At the same time, the simulating circuit newly estimates a displacement of the rotor, performs a comparison in a manner similar to the first rotation time, and obtains a new estimation run-out. However, from the signal of the displacement sensor as a reference for comparison the previous estimated run-out has already been subtracted, therefore, the previous estimated run-out is added to the newly estimated run-out by an adder and the result is set to the estimation run-out at the second rotation time. By repeating the above method, each time the rotor rotates, the estimation run-out is updated and the displacement signal including no run-out is inputted to the compensating circuit. Thus, the noises which are caused by the run-out can be reduced. Preferred embodiments of the invention are described hereinafter with respect to the accompanying drawings which Fig. 1 is a block diagram showing an example of the present invention; Figs. 2A to 2C are diagrams showing a progressing state of the estimation and elimination of a run-out; Figs. 3A and 3B are diagrams showing examples of a feedback circuit for compensation of the run-out; Figs. 4A, 4B, and 4C are diagrams showing examples of a run-out compensating circuit in the case where the elimination of a run-out is limited to a rotational speed component; Fig. 5 is a diagram showing an example of a simulating circuit; Figs. 6A and 6B are diagrams showing examples of spectra of sensor signals before and after the run-out is/was eliminated; and Fig. 7 is a conceptual diagram of a radial bearing control system. Fig. 1 shows a block diagram of an embodiment of the present invention. In Fig. 1, reference numeral 1 denotes the block showing the rotor and 9 indicates a block showing a magnetic bearing controller including the compensating circuit 5, positive/negative switcher 6, power amplifiers 7a and 7b, and electromagnets 3a and 3b in Fig. 7. Reference numeral 10 denotes a block showing a simulating circuit and 11 indicates a block showing a feedback circuit for compensation of a run-out. A force (u) of the magnetic bearing which acts on the rotor corresponds to an input of the block 1 of the rotor. A displacement Xr of the rotor corresponds to an output of the rotor 1. When the blocks 10 and 11 don't exist, a run-out (e) is added to the displacement Xr of the rotor 1 during the operation. The signal is inputted to the magnetic bearing controller 9 in a state in which the run-out component was added to the displacement Xr of the rotor 1. On the basis of the input signal, the magnetic bearing controller 9 decides an output signal so as to position the rotor to a predetermined position (r). Further specifically speaking, the block 9 is a system having the functions of the compensating circuit 5, positive/negative discriminator 6, power amplifiers 7, and electromagnets 3 which have already been described in the conventional technique. The output signal of the block 9 corresponds to a force which is generated by the magnetic bearing. In the conventional control system in which the blocks 10 and 11 don't exist, the magnetic bearing controller 9 controls the rotor in accordance with the displacement signal including the run-out. Therefore, a surplus force which is caused due to a deviation amount that is inherently unnecessary for the control is exerted on the rotor. A run-out elimination when the blocks 10 and 11 are made operative as a feature of the present invention will now be described. The block 10 corresponds to the simulating circuit for mathematically equivalently simulating the input/output characteristics of the rotor 1 and can be constructed by any one of an analog circuit and a digital circuit. The simulating circuit 10 is a state estimating device and traces a displacement X of the rotor 1 at a high fidelity by using the output (u) of the magnetic bearing controller 9 as information. The simulating circuit 10 is mathematically equivalently constructed (however, such a mathematical equivalence is limited to a certain frequency band). Therefore, in the rotor of a magnetic bearing system of the complete floating system, the simulating circuit has poles at an origin and an initial value error increases. Therefore, as shown in Fig. 1, by feeding back a difference between the displacement signal X of the rotor and its estimation signal Xg, the initial errors are converged. An example shown in Fig. 5 relates to a case where it is modeled by using the rotor as one degree of freedom mass point. Fig. 5 shows a construction of the simulating circuit and a feedback control system. A construction of such a circuit is widely applied as an observer theory and a similar simulating circuit can be constructed even by a system of a multidegree of freedom including a bending vibration of the rotor. The block 11 relates to a portion to process an estimation run-out and examples of its processing mechanism are shown in Figs. 2A, 2B, and 2C. First, the simulating circuit 10 estimates a displacement of the rotor 1 by using the output (u) of the magnetic bearing controller 9 as information. An actual displacement sensor signal Xs1 and an estimation displacement which was estimated by the simulating circuit are compared by a subtracter 21, thereby calculating an estimation run-out eg1 (= δ1) at the first time. As for the estimation run-out eg1, by attaching an encoder to the rotor or the like, the value is obtained every rotational angle of the rotor and is stored in the memory. As an estimation run-out eg1 stored in the memory, each amount corresponding to the rotational angle is swept out at the next one rotation of the rotor. By subtracting the stored estimation run-out eg1 from a displacement sensor signal Xs2 by a subtracter 23, a signal X2 from which the run-out was eliminated is obtained. The signal X2 is set to an input signal of the magnetic bearing controller 9, thereby preventing that the controller 9 controls with respect to the run-out component. While the above process is executed, the second run-out estimation is further performed for the next one rotation. In the estimation of the second time, the previous estimation run-out eg1 is added by an adder 22 to a comparison result δ2 between the output of the simulating circuit 10 and the displacement sensor signal X2 from which the run-out was eliminated, thereby obtaining an estimation run-out eg2 of the second time. The second estimation run-out eg2 is also stored into the memory in a manner similar to the previous estimation run-out and is swept out at the time of the next rotation. By repeating such processes, only the signal from which the run-out was always eliminated is inputted to the magnetic bearing controller 9 as shown in Fig. 6. As described above, the block 11 has functions such that while temporarily storing the signal, the signal is swept out with a delay time of one period and the signal of one period before to be stored is added to the signal to be stored. Figs. 3A and 3B show examples of such a circuit construction. In Fig. 3A, reference numeral 13 denotes a delay circuit to delay an output by only a predetermined time. In this case, a delay time L corresponds to the time of one rotation of the rotor, namely, rotating period. When the input signal is inputted to the delay circuit as it is, even as for a deviation which doesn't change with regard to the time like an offset voltage of an operational amplifier, it is regarded as a run-out and is eliminated, so that a gravity compensation of the magnetic bearing controller 9 is deteriorated. Therefore, in Fig. 3A, by providing a high pass filter (feedback circuit of ω/s) 14 before the input section of the delay circuit 13, only the fluctuation component is eliminated. In the case where the estimating mechanism doesn't satisfactorily operate in a very high frequency region because of a limitation of the number of dimensions of the simulating circuit 10, it is sufficient to add a low pass filtering function to the high pass filter as necessary. Fig. 3B shows another example of the feedback circuit 11 for compensation of a run-out. When passing a critical speed, the vibration of the rotor increases due to an unbalance of the rotor system. In this instance, when the above-mentioned run-out eliminating mechanism is operated, even an unbalance vibration is erroneously recognized as a run-out of the displacement sensor, so that there is also considered a case where the vibration of the rotor cannot be suppressed. To avoid such a situation, there is effective means such that the delay time of the delay circuit 13 is set to the time, for example, a half period in which the rotating period of the rotor is divided by a certain integer, thereby reducing an elimination amount of the run-out of the rotational speed component. As another means, there is also a method whereby in case of passing the critical speed, the run-out estimating mechanism is temporarily stopped before it and only the data of the estimation run-out stored in the memory is used for a period of time during which the rotational speed passes the critical speed. As an application of the latter method, the collection of the run-out estimation data is limited to the magnetic bearing control apparatus of the master and the data of the master is stored and used for each of the magnetic bearing control apparatuses of the mass production and only the subtracting process of the run-out is executed, so that the costs can be reduced although a controllability slightly deteriorates. When the user wants to limit a target of the elimination of the run-out to only the rotational speed component, it is desirable to construct the feedback circuit 11 for run-out compensation by combining the delay circuit 13 and tracking filters 15 and 20 as shown in examples of Figs. 4B and 4C. The tracking filter is a filter mechanism which functions synchronously with the rotational speed of the rotor and extracts only the rotational speed component of the input signal. A transfer function of the tracking filter is, for example, as shown in Fig. 4A and is used in the tracking filter 15 in Figs. 4B and 4C. Due to this, the rotating frequency component is efficiently eliminated from the spectrum of the displacement signal. In the above description, nothing is particularly mentioned with respect to the kind of rotating machine. However, for instance, a gas compressor or the like is considered. A gas compressor of the multi-stage type generally uses a structure such that portions near both end portions of the shaft are supported by two bearings and impellers of a plurality of stages are attached to the shaft at the position corresponding to the portion between the bearings. In the above structure, a thrust force is balanced by a pressure of the gas which was compressed by the compressor and a radial load is supported by a radial magnetic bearing. The invention can be applied to such a case. The invention can be also applied to a fluid machine of another type or a rotating electric machine such as a motor or the like.

Claims: A magnetic bearing apparatus having a magnetic bearing for rotatably supporting a rotor (1) by using a magnetic force, comprising : a simulating circuit (10) for calculating an estimated value Xg(t) of rotor displacement on the basis of an output signal u(t) generated by a magnetic bearing controller (9) whereby said output signal u(t) generated by the magnetic bearing controller (9) corresponds to a force generated by the magnetic bearing acting on the rotor (1), a feedback circuit (11) for compensation of run-out, which feedback circuit (11) receives the output δ(t) of a first subtracter (21) and outputs a signal eg(t), a second subtracter (23) for subtracting said output signal eg(t) of the feedback circuit (11) from a displacement signal Xs(t) output from rotor displacement detecting means (2) resulting in a second subtracter output signal X(t), whereby the first subtracter (21) is designed such that it, for generating its output δ(t), subtracts said estimated value Xg(t) of rotor displacement calculated by the simulating circuit (10) from said second subtracter output signal X(t), which second subtracter output signal X(t) is also input into said magnetic bearing controller (9), said feedback circuit (11) is designed such that it comprises a memory means (12) for temporarily storing a value eg(t n ) of the output signal eg(t) of the feedback circuit (11) at a time t n for at least a period L, initially sets the value of its output signal eg(t) equal to the output δ(t) of the first subtracter (21), and subsequently calculates the value eg(t n+1 ) of its output signal at time t n+1 , where t n+1 = t n+L , and where eg(t n+1 ) = eg(t n ) + δ(t n+1 ) , wherein δ(t n+1 ) is the value of the output δ(t) of the first subtracter (21) at time t n+1 . Apparatus according to claim 1, characterized in that the feedback circuit (11) comprises a high pass filter (14) and a delay circuit (13) for delaying the input signal δ(t) by a time of a fraction of an integer of a rotating period of the rotor (1). Apparatus according to claim 2, characterized in that the delay time of said delay circuit (13) is equalized to the rotating period of the rotor (1). Apparatus according to claim 2, characterized in that the high pass filter (14) is a rotational speed sync component extracting filter for ensuring that the output signal eg(t) of the feedback circuit (11) substantially becomes only a rotational speed sync component. Apparatus according to any of the preceding claims, characterized in that the output δ(t) of the first subtracter (21) is fed back as an input to the simulating circuit (10) and the estimated value Xg(t) of the rotor displacement is calculated on the basis of the output signal u(t) generated by the magnetic bearing controller (9) in combination with the output δ(t) of the first subtracter (21). Apparatus according to any of the preceding claims, characterized in that it is designed such that when a critical speed is exceeded, estimation is temporarily stopped and only the data stored in the memory means (12) is used as output signal eg(t) of the feedback circuit (11) for the period of time during which the critical speed is exceeded. Use of a magnetic bearing apparatus as defined by claims 1 to 6 in a rotating machine comprising the rotor (1), and electromagnets (3a, 3b) functioning as stator.

Inventors: FUKUSHIMA YASUO, HIROSHIMA MINORU, MATSUSHISTA OSAMI, TAKAHASHI NAOHIKO, FUKUSHIMA, YASUO, HIROSHIMA, MINORU, MATSUSHISTA, OSAMI, TAKAHASHI, NAOHIKO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1994
Main classification: G05D13
IPC codes: F04D29, F16C32, F16C39